Trucks known as semi-tractors have fifth wheel hitches adapted to lock onto kingpins secured to trailers. The utility of the tractors is limited to trailers equipped with fifth wheel hitch accommodating structures. The load of the trailers is distributed between the trailer wheels and the fifth wheel hitches. The longitudinal location of the fifth wheel hitches relative to the frame of the tractor determines the load distribution on the tractor. The trailers are not mounted directly on the frames of the tractors due to the articulation functions of the fifth wheel hitches. Examples of tractor-trailer combinations having fifth wheel hitches are disclosed by M. Ramun in U.S. Pat. No. 2,860,891; W. W. Grudice in U.S. Pat. No. 3,439,935; G. E. Dodgson in U.S. Pat. No. 3,834,736; R. W. Joyce in U.S. Pat. No. 5,378,007, and N. R. Curry in U.S. Pat. No. 6,231,064.